This application claims the priority of Korean Patent Application Nos. 2002-47358, filed on Aug. 10, 2002 and 2003-16016, filed on Mar. 14, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method and apparatus for rendering an image signal, and more particularly, to a method for rendering an inputted image while minimizing degradation of image quality when the resolution of an inputted image signal is higher than the resolution of a display, and an apparatus for rendering an image signal using the method.
2. Description of the Related Art
When the number of actual pixels of a display is smaller than the number of pixels of an inputted image signal, or when the spatial resolution of the inputted signal is higher than that of the display, the image signal has to be down-sampled so that the entire area of the inputted signal can be presented on the display. However, due to down-sampling of the image signal, a jagged pattern occurs in the portions (for example, text, line, and edge, etc.) of an image with a high spatial frequency (hereinafter, referred to as aliasing), resulting in degraded quality of the image presented on the display.
As a conventional method for solving the above problem, an anti-aliasing method performed on a per-pixel basis has been introduced in “Computer Graphics: Principles and Practice” by Foley et al., Addison Wesley Publishers (2nd edition, 1996). In the method, an image signal is low-pass filtered, smoothened, and is then sampled to reduce the difference in brightness values between neighboring pixels, and to prevent rough image rendering phenomena, such as a jagged pattern. In the method, due to the reduction of the difference in brightness values between pixels, caused by low-pass filtering, blurring that the contrast effect of an image is reduced and the resolution of the image is greatly lowered, occurs.
A method for controlling a boundary between an object and a character on a per-sub-pixel (RGB component) basis is disclosed in U.S. Pat. No. 6,384,839 by Agfa Monotype Corporation. When processing for each sub-pixel, a sudden color transition at a boundary between objects can cause a color fringe error. In the method, the color fringe error is reduced by boundary processing for each sub-pixel, improving the resolution of an image and reducing the difference in brightness between adjacent pixels. In the method, in order to obtain a weight for reducing the brightness difference between adjacent pixels at a boundary between objects, an image is discriminated from a foreground and a background, the foreground and the background are binarized, and the image binarized to a 5-pixel wide window is low-pass filtered, thereby obtaining a weight for the an entire area of the image.
However, the method uses only a horizontal low-pass filter for an RGB-striped structure in a horizontal direction, and performs one-to-one conversion for each pixel, thus the same channel (sub-pixel having the same primary color) must be arranged in a vertical direction. Accordingly, the method is not suitable for adoption in a four-channel display and does not disclose processing, i.e., scaling required in a case where the image resolution is different from the display resolution.